Problem: The scale on a map is 2cm : 8km. If two cities are 10cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 2cm on the map corresponds to an actual distance of 8km. A distance of 10cm on the map is the same as 5 $\cdot$ 2cm. The actual distance between the two cities is 5 $\cdot$ 8km, or 40km.